1. Technical Field of the Invention
The present invention relates to a phased array antenna including a plurality of antenna elements and its phase calibration method for calibrating a phase of the plurality of antenna elements.
2. Related Art
A phased array antenna including a plurality of antenna elements is needed to calibrate a phase of each antenna element in such a manner that radio waves outputted by the antenna elements have the same phase under a predetermined set of conditions.
In the related art, a technique is known to, upon a calibration of a phase of the plurality of antenna elements, (i) change a phase of only one arbitrary antenna element under condition that radio waves with a predetermined power is radiated, (ii) monitor a resultant change in a radiated power of all of the plurality of antenna elements at a receiver located at a front plane side of a radio wave radiation plane to obtain a phase value of the one arbitrary antenna element, (iii) perform the above steps (i) and (ii) for all antenna elements to obtain phase values of all antenna elements, and (iv) calibrate a phase of each antenna elements based on these phase values (see WO2004/013644 A1).
As the other related art, a phase array antenna, which includes a plurality of antenna elements, is known to, in order to maintain low side lobes, dispose an attenuator for an antenna element located at the end portion of an array of the plurality of antenna elements, and to lower an output of radio waves from the antenna element located at the end portion, compared to an antenna element located at the center portion of the array.
However, it is difficult to accurately calibrate the phased array antenna configured to reduce side lobes of the phased array antenna by gradually lowering the output of the radio waves outputted by the phased array antenna from the center portion to the end portion. This is due to the following two reasons.
(1) In the case where the antenna element located at the end portion of the array is calibrated under condition that the radio waves with a predetermined power is radiated by the plurality of antenna elements, when only its phased is changed, a change in the radiated power capable of being detected by the receiver is low because the change is more affected by the other antenna elements and then, a phase of a target antenna element cannot be precisely measured.
(2) The above problem (1) may be solved by obtaining a null point from a directivity pattern of a output difference between two antenna elements of the plurality of antenna elements, and subsequently adjusting phases of the two antenna elements in such a manner that a position of the null point obtained is put at a midpoint between the two antenna elements to calibrate the phases. However, in this case, when the power of the radio waves outputted by the plurality of antenna elements is different every antenna element, as shown in FIG. 6, a depth of the null point (null depth) in the directivity pattern formed by two antenna elements becomes is shallow. Therefore, the null point cannot be precisely detected.